Patent Document 1 describes that compression ignition combustion is carried out in a gasoline engine, of which the geometric compression ratio is set to be equal to or greater than 15. This engine injects, in a heavy load region where the amount of the fuel injected increases, the fuel into a cylinder twice so that the fuel is separately injected during the intake stroke and the compression stroke. The engine allows an air-fuel mixture in a cavity, provided on the top surface of a piston, to compression-ignite in the vicinity of the compression top dead center. Then, the engine allows the air-fuel mixture outside of the recess to ignite and burn, with the intense heat generated by the combustion, during the expansion stroke. This allows compression ignition combustion to be carried out, even in a heavy load region where the amount of the fuel injected increases, without increasing combustion noise.
Patent Document 2 discloses a direct-injection engine having an outwardly-opening fuel injection valve arranged on the center axis of a cylinder, and injecting the fuel in the form of a hollow cone. In this direct-injection engine, the fuel is injected into the cylinder during the second half of a compression stroke, thereby forming, in a combustion chamber, an air-fuel mixture layer and a gas layer surrounding the air-fuel mixture layer. The engine disclosed in Patent Document 2 reduces the cooling loss by making the surrounding gas layer serve as a heat-insulating layer during the combustion of the air-fuel mixture.